1. Field of the Invention
The present invention relates to devices and methods for inspecting wavelength-variable semiconductor lasers that have a wavelength varying function and that are employed for optical communications and second harmonic generation, for example, and methods for inspecting coherent light sources.
2. Description of the Related Art
In recent years, semiconductor lasers having a wavelength varying function have found application in the field of optical communications, for example, and have garnered attention as the fundamental wave for second harmonic generation using nonlinear effects. Distributed feedback (DFB) semiconductor lasers and distributed Bragg reflector (DBR) semiconductor lasers, in which a grating has been integrated onto the semiconductor laser, are semiconductor lasers where the laser can oscillate in a single longitudinal mode. At present, DBR semiconductor lasers and DFB semiconductor lasers are components that are essential for achieving long-distance, high capacity optical communications systems.
As a wavelength variation technique, a method has been proposed in which the oscillation wavelength is tuned by supplying current to the DBR portion of a DBR semiconductor laser to change the refractive index through plasma effects and changes in temperature.
Hereinafter, a DBR semiconductor laser having a wavelength varying function is described (Yokoyama, et al, Transaction of Institute of Electrical Engineers of Japan C, Vol. 120-C, p. 938, 2000). FIG. 14 schematically shows the configuration of an AlGaAs wavelength-variable DBR semiconductor laser with a three-electrode structure.
As shown in FIG. 14, a wavelength-variable DBR semiconductor laser 34 has three regions: an active region 35; a phase control region 36; and a DBR region 37. A method of fabricating the wavelength-variable DBR semiconductor laser 34 thus configured is described briefly below. First, n-type AlGaAs is grown epitaxially on an n-type GaAs substrate using a MOCVD device, after which the active region of the AlGaAs is formed. A p-type AlGaAs is deposited as a cladding layer, and a rib optical waveguide is formed using photolithography. Next, electron beam lithography is used to form a first-order grating (100 nm period) on the optical waveguide. Silicon ions are implanted into the DBR region, where the grating has been formed, and into the phase control change region so as to form a passive optical waveguide. Next, a second crystal growth is performed to deposit p-type AlGaAs as a cladding layer, and then lastly, electrodes for supplying current are formed on the n- and p-sides.
The three-electrode AlGaAs wavelength-variable DBR semiconductor laser has a threshold value of 25 mA, and obtains a 50 mW output with respect to a supply current (operating current) of 150 mA to the active region. FIG. 15 shows the wavelength variability properties when current is supplied to the DBR region. The current supplied to the DBR region (DBR current) was changed to thermally change the refractive index of the DBR region and thereby achieve wavelength variability. The emitted semiconductor laser beam was guided into a light spectrum analyzer and the oscillation wavelength was measured. A stepwise wavelength variation width of 2 nm like that shown in FIG. 15 was obtained with respect to the operation current of 100 mA and a phase current of 0 mA. The oscillation wavelength was maintained in single longitudinal mode even during tuning.
Next, the phase current was set to 20 mA and the wavelength variability properties when the DBR current is similarly changed were measured. Furthermore, the phase current also was set to 40 mA, and the wavelength variability properties when the DBR current is similarly changed were measured. Based on the results that were obtained, the results of the DBR current values experiencing mode hopping (the current values at the points A serving as steps) were plotted in FIG. 16. According to this map, the DBR current (Idbr) and the phase current (Iph) can be controlled and held at a current ratio relationship of Idbr/Iph=0.5, so that it is possible to achieve continuous wavelength variability properties like those shown in FIG. 17.
As described above, the wavelength variability properties are important for wavelength-variable DBR and DFB semiconductor lasers. The factors that are essential for the wavelength-variability properties are: i) single longitudinal mode properties; ii) wavelength variability replicability; and iii) the current ratio Idbr/Iph required for continuous wavelength variability. Single longitudinal mode properties are the most critical aspect for optical communication applications and second harmonic generation, and in second harmonic generation; for example, a large drop in conversion efficiency occurs if the longitudinal mode becomes multimode. Wavelength variability replicability is essential for controlling the wavelength, and as shown in FIG. 15, it has the property of monotonically increasing. In addition, excellent replicability of the wavelength variability properties is essential. The current ratio Idbr/Iph required for continuous wavelength variability must be measured for each semiconductor laser, as there is individual variation between semiconductor lasers.
A light spectrum analyzer or the like was employed conventionally to make measurements when these properties were evaluated, and this required a considerable amount of effort. Accordingly, the simplification of the inspection process was an important issue from the standpoint of mass-producing, for example, wavelength-variable DBR semiconductor lasers.